Multi-channel copper tubing provides the air conditioning and refrigeration industries with a product to create high efficiency heat exchangers. Copper multi-channel tubing offers higher strength and improved thermal conductivity relative to typical aluminum tubing. Copper exhibits antimicrobial properties and superior corrosion resistance, which are desirable in a variety of applications. Such copper tubing can also be easily soldered or brazed and this adds to the ease of heat exchanger construction and repair.
Copper's relatively higher strength is an advantage when it comes to the finished product, but this property also makes it difficult to extrude. During the extrusion operation, the container and die are generally heated to approximately 750° C.-800° C. At 750° C. copper has a flow stress of about 43 MPa at a strain rate of 1 s−1. Accordingly, in high temperature metal forming processes, tooling, such as dies, must have good strength and wear resistance. Hot-work tools steels (e.g., H10, H11, H12, and/or H13) only provide acceptable strength up to about 500° C. In temperatures exceeding 500° C., e.g., greater than 500° C. to about 800° C., metals classified as super alloys demonstrate superior strength compared to tool steels. These super alloys are heat treated to achieve acceptable strengths. Copper extrusion tooling is constructed from commercial super alloys, such as Rene 41, Inconel 718, and ATI 720, due to their favorable strength properties.
However, hot extrusion tooling constructed from super alloys still undergo substantial wear at the elevated temperatures and flow stresses experienced during hot extrusion of copper and/or copper alloys, especially when forming micro-channel tubing. One solution to address the excessive wearing of hot extrusion die assembly components may be to use components constructed of an alloy steel, a super alloy, or other suitable material having a wear resistant coating, such as a hard thin-film of Al2O3, which can be deposited by chemical vapor deposition (CVD), such as that generally disclosed in U.S. Pat. No. 8,191,393, the entirety of which is incorporated herein by reference. However, the CVD coating processing conditions generally degrade the strength and hardness of the base material. Accordingly, subsequent heat treatment step(s) is/are required to restore the desired strength and hardness properties of the base material.
One problem encountered in the heat treatment or hardening process is the loss of adhesion of the wear resistant coating material. Heat treatment for super alloys includes a sequential process of solutionizing, quenching, and aging, which serve to strengthen and harden the super alloy. Solutionizing involves heating the super alloy above its solvus temperature to dissolve soluble intermetallic phases into a solid solution. The material is then quenched to make a supersaturated solid solution, which is followed by heating for a specified duration at a sub-solvus temperature to age the material and produce fine precipitates of intermetallic phases to strengthen and harden the alloy. Following conventional or standard solutionizing conditions for super alloys, delamination of the wear resistant coating was observed during the quenching step.
In view of the foregoing, there is a need for new hot extrusion die tools having wear resistant coatings, as well as viable methods of preparing these coated hot extrusion die tools.